This invention relates to an artificial lung having a built-in heat exchanger. In general in case of conducting an operation to the heart of a patient, it is required on substitute temporarily a blood circulation system outside his body for the function of his heart and lung. The abovementioned blood circulation system which has conventionally been employed comprises an artificial lung for supplying oxygen to the blood and a heat exchanger for controlling the temperature of the blood. However, the conventional artificial lung device is disadvantageous in that because the main body and heat exchanger are formed separately much labour and time are required to sterilize them and connect the component parts by means of conduits to thereby form a blood circulation system outside the patient's body. Further, because even the conduits connecting the component parts are required to be filled with the blood, a great deal of blood is required to allow the blood circulation system outside the patient's body to function satisfactorily.